Wrinkles and image defects are unwanted side effects often encountered in the use of a heated roller fuser in an electrophotographic printer (EP). In typical commercial reproduction apparatus (electrostatographic copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
One type of fuser assembly for typical electrographic reproduction apparatus includes at least one heated roller, having an aluminum core and an elastomeric cover layer, and at least one pressure roller in nip relation with the heated roller. The fuser assembly rollers are rotated to transport a receiver member, bearing a marking particle image, through the nip between the rollers. The pigmented marking particles of the transferred image on the surface of the receiver member soften and become tacky in the heat. Under the pressure, the softened tacky marking particles attach to each other and are partially imbibed into the interstices of the fibers at the surface of the receiver member and then is permanently fixed to the receiver member.
Wrinkles and image defects can be caused by differential overdrive in the fuser nip. Overdrive is caused by deflection of the incompressible elastomer on either or both the fuser roller and pressure roller when the fusing nip is formed and the rollers are rotated. Differences in elastomeric deflection along the axes of the fuser and pressure roller cause corresponding differences in differential overdrive and thus substrate velocity, which in turn cause wrinkles or image defects. Specifically, when the center of the substrate is driven faster than the edges, the trail edge of the substrate will collapse and form wrinkles as the substrate passes through the fuser nip. When the edges of the substrate are driven faster than the center, the trail edge of the substrate will “slap” up or down and smear the image as the image is fused.
Several methods are used to prevent wrinkles and image defects. One common method is to vary the diameter of the fuser or pressure roller along the roller length to reduce the nominal amount of differential overdrive in the nip. Another method is taught in U.S. Pat. No. 5,406,362, where the force that forms the fuser nip is applied inside the ends of one of the rollers in order to impart a bending moment to one of the rollers which in part counteracts the deflection of the fuser and pressure rollers as the nip forming force is applied.
The problem of differential overdrive and resulting wrinkles and image defects is further complicated by temperature differences along the fuser and pressure roller axis, which in turn cause differences in overdrive due to thermal expansion of the elastomer on at least one of the rollers. In addition, the amount of thermal expansion increases during a print run, as heat is continually applied by the fuser lamp(s) to the rollers. Differential thermal expansion is further varied by the width of the substrate. Narrower substrates, as the substrate passes though the fuser nip, causes the ends of the rollers to increase in temperature and thus thermal expansion, since no heat is removed by the substrate outside its path through the fuser nip. The increased thermal expansion of the ends of the roller(s) increases overdrive on the edges of the paper, causing image defects as described.
Another method of improving axial temperature uniformity in a roller fuser is taught in U.S. Pat. No. 6,289,185, where multiple lamps having different filament lengths are used compensate for differences in substrate width. Still another method is taught in U.S. Pat. No. 7,054,572, where the middle of a fuser roller is cooled prior to a print run, to simulate the removal of heat by the substrates, so that axial roller temperatures and resulting differential overdrive is reduced during a subsequent print run.
These methods are not sufficient to prevent all wrinkles and image defects under all conditions, including changes in ambient relative humidity. These problems are especially evident in certain circumstances, such as when heater rollers having thick walls are used to externally heat the fuser roller because the roller transfers heat so well along the axis of the rollers that lamps of different filament length have only a minimal effect on the temperature differential along the fuser roller. Further problems arise due to a lack of access to the middle of the fuser roller because of the placement of other components such as oilers, skives, temperature sensors and cleaners that are necessary for fuser operation.
This controlled fuser system and related method solves these problems by using strategically placed and controlled fluid directed on one of a fuser roller and/or heater rollers such that one or more fusing parameter controls the system, such as cooling air directed at the ends of these rollers based on a receiver sheet width.